FEATURE ARTICLE
The Search for QCD Exotics
Particles predicted by the theory of quantum chromodynamics help explain why the fundamental building blocks of matter are impossible to isolate
Alex Dzierba, Curtis Meyer, Eric Swanson
Virgin Territory
The three of us are working with other experimentalists and theorists (so far about 80 people from 25 institutions in seven countries) to use beams of photons to generate exotic mesons. Why photons? For one, there is very little known about the mesons that high-energy photons can produce. More importantly, QCD theory indicates that such beams should be ideal for creating exotic particles: Quantum mechanically, the photon has a high probability of briefly acting like a virtual quark and antiquark with their spins aligned. Nathan Isgur (then at the University of Toronto) and Jack Paton (University of Oxford) suggested that the flux connecting them can be excited?the string plucked?by collision with a fixed target. This reaction should easily create particles with an exotic value of JPC, the smoking gun in the search for hybrid mesons.
We will be carrying out this experiment at the Thomas Jefferson National Accelerator Facility in Newport News, Virginia, where a large electron accelerator now exists. With some modification, high-energy electrons from this device can be made to produce a beam of suitable photons. The trick is to pass the speeding electrons through a wafer-thin synthetic diamond. As we envision operations, electrons from the accelerator will emerge some 20 feet below ground and slam into the diamond target. Some electrons will be slowed, producing photons by a process called bremsstrahlung, or braking radiation. If things are set up just right, the atoms of the diamond crystal can be made to recoil in step, leading to an enhanced emission at particular energies. This effect (known as coherent bremsstrahlung) has an added bonus: The resulting photons are linearly polarized; that is, their electric fields have a single orientation. This property helps determine the JPC of the mesons produced, whereas the pion beams typically used to generate mesons cannot be polarized.
Although only a fraction of them will produce photons, all the electrons will be swept into the ground. Only photons?about 50 million every second?will reach the target. The detector will consist of a large-aperture superconducting magnet with internal wire chambers and calorimeters as well as a massive lead-glass calorimeter on one end. To keeps costs down, we are recycling: The superconducting magnet, worth about $12 million, was built three decades ago for an experiment at the Stanford Linear Accelerator Center. It was moved in the mid-1980s to the Los Alamos National Laboratory for another study and will soon be transferred to Jefferson Lab for this project. The lead-glass calorimeter was originally built for the Brookhaven experiment at a cost of $3 million and will be brought to the Jefferson Lab as well. If all goes as planned, the first measurements of exotic hybrid particles will be collected within five to seven years. Despite the savings from reusing equipment, the total price tag will still amount to around $30 million.
Clearly, the scale of this project is huge. Yet our experiment is not the only one in the works. New efforts are planned or taking place now in Geneva, in Novosibirsk and in Beijing. And a completely new laboratory for carrying out these kinds of studies is being designed in Japan. We fully expect that a rich suite of QCD exotics will be discovered and thoroughly studied within the next decade. Only then can particle physicists claim to really comprehend glueballs, hybrids, how hadrons decay, how they interact and how mass is generated. Most importantly, only then can we say that we really understand the standard model of matter.
Bibliography
- Close, F. E. 1983. The Cosmic Onion. New York: American Institute of Physics Press.
- Close, F., M. Martin and C. Sutton. 1987. The Particle Explosion. Oxford: Oxford University Press.
- Close, F. E., and P. R. Page. 1998. Glueballs. Scientific American 279:80–85.
- Amsler, C. 1998. Proton-antiproton annihilation and meson spectroscopy with the Crystal Barrel. Reviews of Modern Physics 70:1293-1340.
- Thompson, D. R., et al. 1997. Evidence for exotic meson production in the reaction ??-p ??? ????-pat 18 GeV/c. Physical Review Letters 79:1630-1633.
- Adams, G. S., et al. 1998. Observation of a new JPC = 1?+ exotic state in the reaction ??-p ??? ??+??-??-p at 18 GeV/c. Physical Review Letters 81:5760–5763. [CrossRef]
